Vehicles may be configured with a hybrid propulsion system that utilizes at least two different sources of torque for propelling the vehicle. As one non-limiting example, a hybrid propulsion system for a vehicle may be configured as a hybrid electric vehicle (HEV), wherein one or more electric machines and an internal combustion engine may be selectively operated to provide the requested propulsive effort. Similarly, during deceleration of the vehicle, the electric machines and engine can be selectively operated to provide vehicle driveline braking in order to recapture kinetic energy of the vehicle. In particular, the electric machines can be used to absorb torque that may be converted into electric energy which may be stored in a battery in what may be referred to as regenerative braking. In this way, vehicle efficiency may be increased.
Furthermore, a hybrid propulsion system may include a fixed ratio transmission to transmit torque from the various torque sources to the wheels. A fixed-ratio transmission may have less complexity and reduced cost as compared to a continuously variable transmission. In this way, vehicle complexity and cost may be reduced
In one approach, to extend regenerative braking conditions during deceleration of a vehicle, a state of a fixed-gear transmission may be shifted to a lower gear ratio. By downshifting the transmission, limitations of the energy storage device, engine speed constraints, and/or braking limitations of the electric machines may be avoided.
However, the inventors of the present disclosure have recognized a disadvantage with the above approach. In particular, with a fixed-ratio transmission, the driver can experience driveline disturbances due to abrupt changes in the engine brake torque when a transmission downshift occurs. This can happen during conditions in which engine compression braking is desired in order to decelerate the vehicle. Furthermore, when descending a downhill grade in order to limit the vehicle speed, abrupt driveline torque disturbances can be felt by the driver during engine braking downshifts.
In at least one approach described herein, at least some of the above issues may be addressed by a hybrid propulsion system for a vehicle. The hybrid propulsion system including a multiple step fixed-ratio transmission device for transmitting torque to a first at least one drive wheel, a first electric energy conversion device coupled to an input of the multiple-step fixed-ratio transmission device, a second electric energy conversion device for transmitting torque to a second at least one drive wheel, and a control system, during a vehicle deceleration condition in which the transmission is downshifted, the control system increasing negative torque output of at least one of the first electric energy conversion device and the second electric energy conversion device to meet a desired wheel braking torque in response to the fixed-ratio transmission transitioning from a first gear ratio to a second gear that is lower than the first gear ratio. By increasing the negative torque of the electric energy conversion devices of the hybrid vehicle during an engine braking transmission downshift event, brake torque at the wheels may be increased over a longer period which may result in a smooth torque transmission. In this way, driveline disturbances may be reduced and vehicle drivability may be improved. In other words, by taking advantage of the electric energy conversion devices regenerative braking capabilities, smooth driveline braking can be provided during engine braking transmission downshift events.